A New York gallery has denied claims that a painting it bought for more than $45m (£24.4m) is a fake.
The New York museum called the painting a "masterpiece"
The Metropolitan Museum of Art said it had "no reason to doubt" the authenticity of Madonna and Child, attributed to medieval artist Duccio.
Art history professor James Beck claimed the work, bought at a private sale handled by the Christie's auction house, was an "unpleasant" forgery.
Professor Beck believes the painting dates from the late 19th Century.
"As a taxpayer, I regret that $50m was thrown in the toilet," he said.
In a statement, however, the museum said the painting has been "carefully examined... as a matter of course".
Christie's, meanwhile, said it was "entirely satisfied" with the artwork's attribution.
Professor Beck claims the positioning of a parapet in front of the Madonna to create a "second plane" in the work is the biggest clue to its forgery.
The technique was not used by artists until 100 years after Duccio is said to have finished the painting.
Measuring 8in by 11in (20cm by 28cm), the Madonna and Child is reportedly the most expensive painting ever purchased by the museum.
Born in Siena around 1260, Duccio di Buoninsegna was the foremost painter of early Renaissance Siena.
Only around a dozen of his works are known to have survived.